FIG. 1 shows a conventional joint between a stringer foot 1 and a wing skin panel 2. The components are bonded together by an adhesive layer 3. A failsafe bolt 4 maintains a connection between the components in the event of failure of the adhesive layer 3.
Three main types of load typically act on the joint when the aircraft wing is in use. Firstly, loads act on the joint as a result of local curvature of the wing skin 2 which is caused by global bending of the wing. Secondly, shear loads act on the wing skin 2 which must be transmitted to the stringer 1 through the joint. Finally, as a result of the geometrical discontinuity at the stringer run-out 5, peeling loads can act to separate the stringer foot 1 from the skin 2.
Shear stresses 6 are transferred by the adhesive layer 3 from the skin 2 to the stringer foot 1 as shown in the graph 7. The shear stresses are at a maximum where the transfer starts, then they reach zero once all the load is proportionally distributed between the components. While the adhesive layer 3 remains intact, the bolt 4 transmits little or no shear stress.
A problem with the arrangement of FIG. 1 is that the adhesive layer 3 is susceptible to cracking which typically initiates at the stringer run-out 5 where the shear stress 7 is at a maximum.
Although the structure as a whole can withstand relatively high loads before catastrophic failure due to the presence of the failsafe bolt 4, aircraft regulations require the total integrity of the structure to be preserved. It is therefore necessary to prevent crack initiation up to ultimate load levels, that is the highest load levels that are likely to be experienced during the operational life of the aircraft. This is typically achieved by thickening the skin 2.